OBJECTIVES: Reducing unnecessary antibiotic use, particularly among children, continues to be a public health priority. Previous intervention studies have been limited by size or design and have shown mixed results. The objective of this study was to determine the impact of a multifaceted, community-wide intervention on overall antibiotic use for young children and on use of broad-spectrum agents. In addition, we sought to compare the intervention's impact on commercially and Medicaid-insured children. METHODS: We conducted a controlled, community-level, cluster-randomized trial in 16 nonoverlapping Massachusetts communities, studied from 1998 to 2003. During 3 years, we implemented a physician behavior-change strategy that included guideline dissemination, small-group education, frequent updates and educational materials, and prescribing feedback. Parents received educational materials by mail and in primary care practices, pharmacies, and child care settings. Using health-plan data, we measured changes in antibiotics dispensed per person-year of observation among children who were aged 3 to <72 months, resided in study communities, and were insured by a participating commercial health plan or Medicaid. RESULTS: The data include 223,135 person-years of observation. Antibiotic-use rates at baseline were 2.8, 1.7, and 1.4 antibiotics per person-year among those aged 3 to <24, 24 to <48, and 48 to <72 months, respectively. We observed a substantial downward trend in antibiotic prescribing, even in the absence of intervention. The intervention had no additional effect among children aged 3 to <24 months but was responsible for a 4.2% decrease among those aged 24 to <48 months and a 6.7% decrease among those aged 48 to <72 months. The intervention effect was greater among Medicaid-insured children and for broad-spectrum agents. CONCLUSIONS: A sustained, multifaceted, community-level intervention was only modestly successful at decreasing overall antibiotic use beyond substantial secular trends. The more robust impact among Medicaid-insured children and for specific medication classes provides an argument for specific targeting of resources for patient and physician behavior change.